It will be appreciated that a number of attempts have been made to determine the trajectory of an incoming round or bullet in order to be able to isolate the shooter and to provide what other measures are necessary in order to neutralize the threat. To do so, it has been the practice to deploy an array of acoustic sensors to acoustically sense the location of the source of an incoming projectile. This is usually done by detecting muzzle blast and the shock wave that attends it.
However, there are a number of problems involved in ascertaining the source of the projectile acoustically, mainly centering around the arrival of reverberations. Moreover, it is a practice of professional snipers to prefer subsonic rounds that acoustic methods do not detect. Additionally, in a firefight or when there are multiple rounds being fired, it is virtually impossible to detect which individual sound trails belong to which shooters. The result is that acoustic means are not particularly useful in identifying the shooter or his location.
There is also another problem associated with acoustic determination of the position of a projectile and that is the fact that the shock wave extends out from the forward portion of the projectile some distance behind it. Thus, the position of closest approach to an acoustic sensor is not easily ascertained. As a result, it is not easy to determine what part of the shock wave has been sensed in order to ascertain source location. This inability to specify closest approach impacts the time delay measurement between the sensors that is used to triangulate on the position of the projectile. A further problem with acoustic sensing is that the speed of sound plays a factor so that pinpointing a projectile and determining its trajectory is either delayed or requires a relatively large baseline for the acoustic array sensors.
This baseline in and of itself is problematic due to the large array configuration. It is unwieldy and very visible. For instance, when carried on a vehicle, the array extends above the vehicle and can be seen for hundreds of yards, making it easily avoidable by shooters.
Additionally, the large arrays such as acoustic arrays are not easily man-portable. When an individual attempts to erect an acoustic array, for instance in a foxhole, not only do the confines of the foxhole make deployment awkward, the presence of the individual is readily ascertainable due to the bulky array that is projected above the foxhole.
Not only is it important to be able to ascertain the trajectory of incoming fire from the point of view of the individual soldier, if one could mount a convenient device on a vehicle, one could at least tell the quadrant from which incoming fire is coming in so that one could bail out at the opposite side of the vehicle.
It will be appreciated that, with respect to the type of acoustic sensors that are vehicle mounted, the acoustic array is usually mounted on a pole over the top of the vehicle. The problem with the acoustic sensors, as mentioned above, is that the enemy quickly learns which ones to shoot at and which ones not to. Thus it is a requirement that whatever system is deployed, the one that has the lowest observability is preferred, so that the enemy does not know which vehicles or individuals are equipped and which are not.
As to aircraft, remotely controlled vehicles or unmanned aerial vehicles, UAVs, oftentimes are fired at by small arms fire, which disables them without the knowledge of ground controllers. Oftentimes the only time that a ground controller is aware that a UAV has been hit is by malfunction in the telemetry or in fact a ceasing of telemetry operations. The ground controller has no way of sensing incoming fire in order to have the aircraft take evasive action. It is for this reason that it is desirable to provide a lightweight, compact and extremely energy-stingy trajectory sensing system that is UAV-mountable.
By way of further background, it has been ascertained that the naturally occurring electric field surrounding a moving charged object is changed by the passage of the charged object through the electric field; and that this change can be sensed by a so-called E-field sensor. Such an E-field sensor is described in provisional U.S. Patent Application 60/640,465 filed Dec. 31, 2004 by Paul A. Zank, Eldon Sutphin, David Buchanan, and George Succi, entitled “Method and Apparatus for Detecting Individuals Using Electrical Field Sensors,” assigned to the assignee hereof and incorporated herein by reference. The E-field sensor basically senses the change in the electric field due to a moving charged body and was originally used to detect the charged particles that result from rocket propulsion of a missile.
Moreover, as described in U.S. patent application Ser. No. 11/104,125 filed Apr. 12, 2005 by Paul A. Zank and Eugene S. Rubin, assigned to the assignee hereof and incorporated herein by reference, E-field sensors have been used to detect power lines in wire strike avoidance systems in which differential E-field sensors are used.
E-field sensors of the type described have also been used to isolate lightning strikes and have been used for other purposes.